William Stallings
Data and Computer
Communications
Chapter 1
Introduction
A Communications Model
Source
generates data to be transmitted
Transmitter
Converts data into transmittable signals
Transmission System
Carries data
Receiver
Converts received signal into data
Destination
Takes incoming data
Simplified Communications
Model - Diagram
Key Communications Tasks
 Transmission System Utilization
 Interfacing
 Signal Generation
 Synchronization
 Exchange Management
 Error detection and correction
 Addressing and routing
 Recovery
 Message formatting
 Security
 Network Management
Simplified Data
Communications Model
Networking
Point to point communication not usually
practical
Devices are too far apart
Large set of devices would need impractical number
of connections
Solution is a communications network
Simplified Network Model
Wide Area Networks
Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)
Circuit Switching
Dedicated communications path established for
the duration of the conversation
e.g. telephone network
Packet Switching
Data sent out of sequence
Small chunks (packets) of data at a time
Packets passed from node to node between
source and destination
Used for terminal to computer and computer to
computer communications
Frame Relay
Packet switching systems have large overheads
to compensate for errors
Modern systems are more reliable
Errors can be caught in end system
Most overhead for error control is stripped out
Asynchronous Transfer Mode
ATM
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet switching
technique
Integrated Services Digital
Network
ISDN
Designed to replace public telecom system
Wide variety of services
Entirely digital domain
Local Area Networks
Smaller scope
Building or small campus
Usually owned by same organization as
attached devices
Data rates much higher
Usually broadcast systems
Now some switched systems and ATM are being
introduced
Protocols
Used for communications between entities in a
system
Must speak the same language
Entities
User applications
e-mail facilities
terminals
Systems
Computer
Terminal
Remote sensor
Key Elements of a Protocol
Syntax
Data formats
Signal levels
Semantics
Control information
Error handling
Timing
Speed matching
Sequencing
Protocol Architecture
Task of communication broken up into modules
For example file transfer could use three
modules
File transfer application
Communication service module
Network access module
Simplified File Transfer
Architecture
A Three Layer Model
Network Access Layer
Transport Layer
Application Layer
Network Access Layer
Exchange of data between the computer and
the network
Sending computer provides address of
destination
May invoke levels of service
Dependent on type of network used (LAN,
packet switched etc.)
Transport Layer
Reliable data exchange
Independent of network being used
Independent of application
Application Layer
Support for different user applications
e.g. e-mail, file transfer
Addressing Requirements
Two levels of addressing required
Each computer needs unique network address
Each application on a (multi-tasking) computer
needs a unique address within the computer
The service access point or SAP
Protocol Architectures and
Networks
Protocols in Simplified
Architecture
Protocol Data Units (PDU)
At each layer, protocols are used to
communicate
Control information is added to user data at
each layer
Transport layer may fragment user data
Each fragment has a transport header added
Destination SAP
Sequence number
Error detection code
This gives a transport protocol data unit
Network PDU
Adds network header
network address for destination computer
Facilities requests
Operation of a Protocol
Architecture
TCP/IP Protocol Architecture
Developed by the US Defense Advanced
Research Project Agency (DARPA) for its packet
switched network (ARPANET)
Used by the global Internet
No official model but a working one.
Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer
Physical Layer
Physical interface between data transmission
device (e.g. computer) and transmission
medium or network
Characteristics of transmission medium
Signal levels
Data rates
etc.
Network Access Layer
Exchange of data between end system and
network
Destination address provision
Invoking services like priority
Internet Layer (IP)
Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end systems and routers
Transport Layer (TCP)
Reliable delivery of data
Ordering of delivery
Application Layer
Support for user applications
e.g. http, SMPT
TCP/IP Protocol Architecture
Model
OSI Model
Open Systems Interconnection
Developed by the International Organization for
Standardization (ISO)
Seven layers
A theoretical system delivered too late!
TCP/IP is the de facto standard
OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
OSI v TCP/IP
Standards
Required to allow for interoperability between
equipment
Advantages
Ensures a large market for equipment and software
Allows products from different vendors to
communicate
Disadvantages
Freeze technology
May be multiple standards for the same thing
Standards Organizations
Internet Society
ISO
ITU-T (formally CCITT)
ATM forum
Further Reading
Stallings, W. Data and Computer
Communications (6th edition), Prentice Hall
1999 chapter 1
Web site for Stallings book
www.shore.net/~ws/DCC6e.html
Web sites for IETF, IEEE, ITU-T, ISO
Internet Requests for Comment (RFCs)
Usenet News groups
comp.dcom.*
comp.protocols.tcp-ip